Field
This disclosure relates to treatment of wastewaters containing organic matter, phosphorus and nitrogen. In particular, the disclosure relates to utilizing sulphur compounds as the electron carrier for biological nutrient removal of wastewater treatment
Background
Since the discovery of activated sludge process and the introduction of the biological nutrient removal processes, the biological Phosphorus (P), Nitrogen (N) and Carbon (C) removal processes has remained the same, i.e., with electron flow from carbon to oxygen through heterotrophic carbon oxidation, as shown in FIG. 1.
The details of this biological P, N and C removal process are as follows:                Reactor 1: P is released and organic carbon is taken up and stored as poly-hydroxyalkanoates (PHAs) by the Poly-phosphate Accumulating Organisms (PAOs) when no oxygen or nitrate is present.        Reactor 2: When nitrate is present, the stored organic carbon is oxidized to CO2 through heterotrophic denitrification. Nitrate is reduced to N2. Electron flows from organic carbon to nitrate with simultaneous P-uptake by the PAOs.        Reactor 3: Electron flows from ammonia to oxygen with nitrate formed through autotrophic nitrification which is recycled back to Reactor 2.        
If nitrogen removal is not necessary, the biological processes can be simplified as FIG. 2. The biological processes are as follows:                Reactor 1: P is released and organic carbon is taken up and stored as PHAs by the PAOs when no oxygen is present.        Reactor 2: When oxygen is present, the stored organic carbon is oxidized to CO2 through heterotrophic carbon oxidation. Electron flows from organic carbon to oxygen with simultaneous P-uptake by the PAOs.        
As the heterotrophic carbon oxidation and heterotrophic denitrification process has a very high sludge yield factor, depending on the sludge age, about 40-50% of the organic carbon in the sewage will be converted to CO2 while the rest converted to sewage sludge. The disposal of excess sludge, which often involves sludge digestion, dewatering and incineration, is not only costly, but also unwelcome by neighbours to the facility.
Since the introduction of biological phosphorus (P) removal process in 1970s, the process has relied on the electron flow from organic carbon to oxygen via an integrated P-uptake and release cycle. As the process has a high sludge yield, excess sludge disposal is required.
Sulphate Reduction Autotrophic Denitrification and Nitrification Integrated SANI Process
Making use of the sulphate ion available in the saline sewage of Hong Kong, where seawater is used for toilet flushing, the Hong Kong University of Science and Technology developed the novel Sulphate reduction Autotrophic denitrification and Nitrification Integrated (SANI) process (Lau et al., 2006; Lu et al., 2009; Wang et al., 2009) as shown in FIG. 3. In the SANI process, sulphate originating from seawater is used to oxidize organic carbon to CO2 while sulphate is reduced to dissolved sulphide by the sulphate reduction bacteria in the first reactor. On the other hand, nitrogen present in ammonia is oxidized by oxygen to nitrate in the third reactor by the autotrophic nitrifiers. The nitrate formed is then recycled to the second reactor to react with the sulphide ion to convert into nitrogen gas by the autotrophic denitrifiers while sulphide will be converted back to sulphate ion. An example of the SANI process is described in PCT/CN2011/002019 filed 2 Dec. 2011, published as WO 2012/071793 A1, and as correspond application US2013/0256223.
Each liter of seawater contains about 2.7 grams of sulphate. When used with a seawater flushing system, the sulphates in seawater can be used to oxidize the organic carbon pollutants forming sulphide; while the sulphide formed can then be used to reduce nitrate to nitrogen gas through autotrophic denitrification, which can help sludge reduction. The SANI process uses sulphate-reducing bacteria to oxide and eliminate pollutants in the seawater-mixed sludge. It is noted, however, that the sulphate cannot directly reduce sludge; however, it is used as a oxidizing and reducing agent to remove organic carbon and nitrate, which in turn results in sludge reduction.
The three key biological chemical processes all produce minimal sludge as shown in the following equations:                (1) Heterotrophic Sulphate Reduction:127.8 g COD+192 g SO42−+55.8 g H2O→68 g H2S+2.4 g Sludge+244 g HCO3−        (2) Sulfide Sulphide Oxidation and Autotrophic Denitrification:124 g NO3−+7.32 g HCO3−+44.54 g H2S→28 g N2+125.76 g SO42−+2.66 g Sludge        (3) Autotrophic Nitrification:18 g NH4++1.32 g CO2+62.4 g O2→0.94 g Sludge+62 g NO3−+2 g H++17.64 g H2O        
Limited-Oxygen Sulphur Cycle-Associated EBPR (LOS-EBPR) Process
In view of its significant environmental and financial benefits to minimize sludge production by the SANI process, research has been conducted to extend the SANI process for P-removal. The success of this biological P-removal SANI process lies with the development of the P uptake and release in the sulphur cycle. Although the oxygen and nitrate induced P-uptake and release phenomenon has been fully studied and understood, the proposed sulphur cycle involved P-uptake and release has not been extensively tested. The phenomenon is described in Sulphate Reducing Bacteria (SRB) with the PAOs.